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Title: ELEVATED ATMOSPHERIC CO2 EFFECTS AND SOIL WATER FEED-BACKS ON SOIL RESPIRATION COMPONENTS IN A COLORADO GRASSLAND

Author
item PENDALL, E - UNIVERSITY OF WYOMING
item DEL GROSSO, S - COLORADO STATE UNIVERSITY
item King, Jennifer
item Lecain, Daniel
item MILCHUNAS, D - COLORADO STATE UNIVERSITY
item Morgan, Jack
item Mosier, Arvin
item OJIMA, D - COLORADO STATE UNIVERSITY
item PARTON, W - COLORADO STATE UNIVERSITY
item TANS, P - UNIVERSITY OF COLORADO

Submitted to: Global Biogeochemical Cycles
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/20/2002
Publication Date: 5/15/2003
Citation: Pendall, E., Del Grosso, S., King, J.Y., Lecain, D.R., Milchunas, D.G., Morgan, J.A., Mosier, A.R., Ojima, D.S., Parton, W.A., Tans, P.P. 2003. Elevated atmospheric CO2 effects and soil water feed-backs on soil respiration components in a colorado grassland. Global Biogeochemical Cycles. 17:1-13.

Interpretive Summary: Rising concentrations of atmospheric carbon dioxide have prompted the initiation of numerous studies designed to evaluate how higher carbon dioxide will affect important world ecosystems. Only a handful of long-term field studies have yet investigated the impact of rising carbon dioxide on unperturbed native ecosystems, and fewer yet have bothered to attack the difficult yet important issue of how rising carbon dioxide will influence important soil biological processes that ultimately determine how C is stored in terrestrial ecosystems. This study evaluates the impact of a doubling of carbon dioxide on soil biological processes in the shortgrass steppe, an important grassland on the western border of the North American Great Plains. The results suggest that while doubling carbon dioxide significantly increases photosynthesis and growth of native grasses, it also increases the rates of soil carbon loss through respiration, which tends to lessen or even negate one of the potential positive benefits of carbon dioxide enrichment, enhanced plant production. These results suggest that elevated carbon dioxide may not necessarily enhance the ability of native ecosystems like the shortgrass steppe to store carbon.

Technical Abstract: The shortgrass steppe is a semi-arid grassland, where elevated CO2 reduces stomatal conductance and promotes soil moisture storage. Enhanced biomass growth from elevated CO2 has been attributed in part to soil moisture effects. However, the influence of this soil moisture feedback on C cycling has received little attention. We used open-top chambers to increase atmospheric CO2 concentrations to twice-ambient for four growing seasons. Soil respiration rates and stable C isotopes of soil CO2 were measured during the third and fourth seasons. Elevated CO2 increased soil respiration rates by ~25% in a moist growing season and by ~85% in a dry season. Stable C isotope partitioning of soil respiration into its components of decomposition and rhizosphere respiration was facilitated on all treatments by a 13C disequilibrium between currently growing plants and soil organic matter. Decomposition rates were more than doubled by elevated CO2, whereas rhizosphere respiration rates were not changed. In general, decomposition rates were most significantly correlated with soil temperature, and rhizosphere respiration rates were best predicted by soil moisture content. Model simulations suggested that soil moisture feedbacks, rather than differences in substrate availability, were primarily responsible for higher total respiration rates under elevated CO2. By contrast, modeling demonstrated that substrate availability was at least as important as soil moisture in driving CO2 treatment differences in soil organic matter decomposition rates.